岩石学报  2013, Vol. 29 Issue (3): 756-768   PDF    
藏南拉轨岗日变质核杂岩核部花岗质片麻岩的地球化学特征及构造意义
辜平阳1, 何世平1, 李荣社1, 时超1, 董增产1, 査显锋1, 吴继莲2, 王轶1     
1. 国土资源部岩浆作用成矿与找矿重点实验室,西安地质调查中心,西安 710054;
2. 长安大学地球科学与资源学院,西安 710054
摘要: 拉轨岗日变质核杂岩花岗质片麻岩高SiO2、K2O,低Fe2O3、MgO、MnO,铝饱和指数A/CNK为1.07~1.19,含白云母、石榴子石等过铝质矿物,属高钾钙碱性过铝质岩石。稀土总量较高,轻稀土富集,轻、重稀土分馏明显,Eu负异常;富集大离子亲石元素Rb、Th、U,贫高场强元素Sr、Nb,Hf和Ba亏损,具有壳源S型花岗岩的特征。87Sr/86Sr初始比值ISr变化于0.709~0.7306,εNd(t)值较低,为-7.13~-8.97,且εNd(t)和ISr无明显相关关系,εSr(t)值较高,为80.65~379。(206Pb/204Pb)t=18.1062~18.8085,(207Pb/204Pb)t=15.6713~15.7901,(208Pb/204Pb)t=37.529~38.1815,表现出地壳特征的Sr、Nd、Pb同位素特征。岩浆源区可能是以粘土岩主,砂质岩占次要地位的沉积岩,经部分熔融形成的花岗质岩浆上升侵位形成。LA-ICPMS锆石U-Pb年龄为514Ma,是泛非造山事件在北喜马拉雅拉轨岗日一带的地质记录。岩石形成于由碰撞造山的挤压环境向后碰撞造山的伸展环境转化阶段,说明泛非碰撞造山事件在拉雅拉轨岗日一带可能结束,进入后碰撞造山的构造演化阶段。岩浆底侵可能使拉轨岗日变质核杂岩在该阶段已经初步隆起,对变质核杂岩的形成起了重要作用。
关键词: 拉轨岗日变质核杂岩     花岗片麻岩     地球化学     锆石U-Pb定年     泛非造山事件    
Geochemical features and tectonic significance of granitic gneiss of Laguigangri metamorphic core complexes in southern Tibet
GU PingYang1, HE ShiPing1, LI RongShe1, SHI Chao1, DONG ZengChan1, ZHA XianFeng1, WU JiLian2, WANG Yi1     
1. Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, MLR, Xi'an Geological Survey Center, Xi'an 710054, China;
2. Earth Science & Resources College of Chang'an University, Xi'an 710054, China
Abstract: The granitic gneiss of Laguigangri metamorphic core complexes in southern Tibet is characterized by high SiO2 and K2O, low Fe2O3, MgO and MnO, with alumina saturation index (A/CNK) of 1.07~1.19 and peraluminous minerals (such as muscovite and garnet). These features suggest that the granitic gneiss belongs to the high-K calci-alkaline peraluminous rocks. It is obvious that the chondrite-normalized REE shows right inclined patterns with obvious negative Eu abnormity, simultaneously, the primitive mantle normalized trace element spidergrams are characterized by enriched LILE (such as Rb, Th and U) and depleted HFSE (such as Sr and Nb), as well as evidently low contents of Ba and Hf, which are compatible with the characteristics of crustal derived S-type granites. The initial values of 87Sr/86Sr (ISr) are in a range of 0.709~0.7306, the εSr(t) in a range of 80.65~379 and εNd(t=514Ma) values of -7.13~-8.97, without significant correlation between εNd(t) and ISr. The ratios of (206Pb/204Pb)t(=18.1062~18.8085), (207Pb/204Pb)t(=15.7~15.8) and (208Pb/204Pb)t(=37.5~38.2) also reveal crustal Pb isotopic features. Therefore, the granitic magma is originated from partially melting of clay and sandstone sedimentary rocks. With the U-Pb isotopic age of 514Ma, we believe that these rocks in Laguigangri formed during the transition period from syncollision (compressional) to post collision (extensional environment) in Pan African orogenic events in the north Himalayan. This magmatic activity may play an important role in formation of the Laguigangri metamorphic core complex.
Key words: Laguigangri metamorphic core complexes     Granitic gneiss     Geochemistry     Ziron U-Pb dating     Pan African orogenic events    

变质核杂岩(MCC)是20世纪美国学者在研究北美西部科迪勒拉造山带中一套独特的伸展构造和岩石组合的基础上提出来的,由变质核和侵入岩组成的穹形或拱形孤立隆起,其上为构造滑脱和扩张的不变质盖层组成,是地壳较深层次地质信息的载体,研究地球深部信息的“窗口”(Davis and Coney, 1979Coney,1980)。随着研究的深入,中外地质学家先后报道了一系列不同时代、不同构造特色的变质核杂岩(郑亚东和张青,1993宋鸿林,1995Faure et al., 1996孙岩等,1997Wang et al., 2001Darby et al., 2001Yan et al., 2003李德威等,2003赵海滨等,2007关会梅等,2008)。在变质核杂岩的形成时代、成因、伸展构造与岩浆活动及地壳减薄机制等方面取得重要进展(Davis和郑亚东,2002宋鸿林,2002刘德民,2003a楼法生等,2005沈晓明等,2008苏春乾等,2008)。变质核杂岩往往是多期岩浆活动的中心,且空间结构及拆离滑脱构造系的发育型式等为成矿元素的活化、迁移、富集提供了良好的条件,对于指导核杂岩区的找矿具有重要意义(马寅生等,2002付伟等,2005尹维青和李建旭,2007)。

前人对拉轨岗日变质核杂岩的精细结构、变形变质、形成机制、遥感影像特征及同构造侵入体与变质核杂岩的形成关系进行了详细研究(隋志龙等, 2003, 2006李德威等, 2003, 2004刘德民,2003b袁晏明等,2003张金阳等,2003李建忠等,2005),但对该杂岩的地球化学特征、成因及构造属性等探讨较少,其形成时代仍有争议。拉轨岗日变质核杂岩带东部的康马岩体已获得485Ma的Rb-Sr等时线年龄(王俊文等,1981),锆石U-Pb年龄集中于485~558Ma (许荣华和金成伟,1986Lee et al., 2000刘文灿等,2004b夏斌等,2008),拉轨岗日变质核杂岩阿马花岗质片麻岩曾有1.8~1.9Ga (中国地质大学,2003)和1811.7±7.2Ma的SHRIMP锆石U-Pb年龄(廖群安等,2007)报道。鉴于此,在对阿马一带拉轨岗日变质核杂岩核部花岗质片麻岩有必要进行详细的地球化学和高精度同位素定年研究,进而探讨花岗质片麻岩形成时代及大地构造意义。

①中国地质大学. 2003. 1:25万定日县幅区域地质调查报告

1 区域地质背景

拉轨岗日变质核杂岩带位于藏南低分水岭一带,与高喜马拉雅隆起带之间以定日-岗巴拗陷带分隔,表现为由一系列核部发育花岗岩体的具有伸展构造性质的热穹窿组成的隆起带,是青藏高原南部典型的变质核杂岩之一。区内出露前震旦纪拉轨岗日杂岩(AnZL)和抗青大岩组(AnZk)、石炭纪少岗群(CS)、早二叠世比聋组(P1b)、中二叠世康马组(P2k)、中二叠世白定浦组(P2b)、早-中三叠世吕村组(T1-2l)、晚三叠世涅如组(T3n)、早中侏罗世日当组(J1-2r)及第四系,白云母二长花岗岩及片麻状黑云二长花岗岩后期侵入(图 1)。本文以阿马一带拉轨岗日变质核杂岩为研究对象,该核杂岩不同程度地被后期NNE向左行平移断层切割,但仍显示典型的三层结构型式。

图 1 藏南定日县长所乡北拉轨岗日一带地质图(据李德威等,2003) 1-震旦纪拉轨岗日变质杂岩;2-震旦纪抗青大岩组;3-石炭纪少岗群;4-早二叠世比聋组;5-中二叠世康马组;6-中二叠世白定浦组;7-早-中三叠世吕村组;8-晚三叠世涅如组;9-早-中侏罗世日当组;10-第四系上更新统;11-片麻状黑云二长花岗岩;12-白云母二长花岗岩;13-古生界-新生界;14-震旦纪马卡鲁变质杂岩;15-石炭系-侏罗系;16-花岗岩体;17-平移断层;18地质界线;19-采样位置 Fig. 1 Geological sketch map of Laguigangri in northern Changsuo village, Dingri County, southern Tibet (after Li et al., 2003) 1-Sinian Laguigangri metamorphic complexes; 2-Sinian Kangqingda Rock Formation; 3-Carboniferous Shaogang Group; 4-Lower Permian Bilong Formation; 5-Middle Permian Kangma Formation; 6-Middle Permian Kangma Baidingpu Formation; 7-Early-Middle Triassic Lucun Formation; 8-Late Triassic Nieru Formation; 9-Early-Middle Jurassic Ridang Formation; 10-the Upper Pleistocene of Quaternary; 11-gneissic biotite monzogranite; 12-muscovite monzogranite; 13-Paleozoic-Cenozoic; 14-Sinian Makalu metamorphic complexes; 15-Paleozoic-Cenozoic; 16-granite; 17-strike-slip fault; 18-geological boundary; 19-sample location

变质核分布在拉轨岗日隆起带的主脊部位,主体由拉轨岗日群变质杂岩组成,该群可分解为两个部分,彼此之间以正断式韧性剪切带接触。上部抗青大岩组,主要岩石类型包括云母石英片岩、十字石蓝晶石片岩、石榴云母片岩、石英岩和大理岩;下部拉轨岗日杂岩,以片麻岩为主,主要岩石类型为眼球状片麻岩、条痕状片麻岩、条带状片麻岩、花岗质片麻岩和混合花岗岩,局部夹有透镜状斜长角闪岩、榴闪岩。变质核近中心部位出现近等轴状分布的花岗岩体,变质核中的花岗岩体边缘片麻理发育,片麻理与接触带平行,内部含有片麻岩的捕掳体。强烈变质变形的变质核基底岩系与盖层之间为大型滑脱断层,由基底拆离断层及其相关的正断式韧性剪切带组成。拉轨岗日变质核杂岩的盖层系统由石炭系、二叠系、三叠系和侏罗系浅变质或未变质的岩石组成,围绕变质核由老至新呈环形分布。受基底拆离断层伸展拆离的影响,在局部地段盖层下部地层出现尖灭和缺失现象。盖层明显地受变质核内岩浆侵入及其相关的热活动的影响,发育次级拆离断层和脆性正断层,拆离断层沿着能干性不同的岩性界面顺层发育,主要是沿着大套碳酸盐岩与碎屑岩的界面,以中等到低角度产出,断层面在地表的倾角一般为30°~45°,带动核部变质杂岩上升,导致其揭顶和剥露,引起基底与盖层之间滑脱拆离。

2 花岗片麻岩年代学特征 2.1 样品及其岩相学特征

用于同位素测年的样品07NR-2采自西藏南定日县长所乡北阿马一带,拉轨岗日变质核杂岩核部(地理坐标:N=32°01′30.5″;E=91°41′28.5″;H=2519m),岩性为眼球状黑云母花岗片麻岩。样品重量约25kg,岩石为浅灰色,粗粒鳞片花岗变晶结构,片麻状构造、眼球状-条带状构造,矿物组成为:石英(20%~25%)、斜长石(30%~35%)、钾长石(10%~15%)、黑云母(15%~20%),石榴子石(1%~3%),白云母(1%~2%),副矿物有锆石和磷灰石。

2.2 分析方法

锆石的阴极发光(CL)图像在西北大学扫描电镜实验室完成,采用FEI公司XL30型SFEG电子束进行锆石内部结构显微照相分析。测试点的选取首先根据锆石反射光和透射光照片进行初选,再与CL图像反复对比,力求避开内部裂隙和包裹体,以获得较准确的年龄信息。

LA-ICP-MS法锆石微区U-Pb年龄测定在西北大学大陆动力学国家重点实验室的Agilent7500型ICPMS和德国Lambda Physik公司的ComPex102 ArF准分子激光器(工作物质ArF,波长193nm)以及MicroLas公司的GeoLas200M光学系统的联机上进行。激光束斑直径为30μm,激光剥蚀深度为20~40μm。实验中采用He作为剥蚀物质的载气,用美国国家标准技术研究院研制的人工合成硅酸盐玻璃标准参考物质NIST SRM610进行仪器最佳化,采样方式为单点剥蚀,数据采集选用一个质量峰一点的跳峰方式,每完成4~5个待测样品测定,插入测标样一次。在所测锆石样品15~20个点前后各测2次NIST SRM610。锆石年龄采用标准锆石91500作为外部标准物质,元素含量采用NIST SRM610作为外标。由于SiO2在锆石中的含量较恒定,选择29Si作为内标来消除激光能量在点分析过程中以及分析点之间的漂移,对于大多数元素单点分析的相对标准偏差为5%~15%。详细分析步骤和数据处理方法参见相关文献(Gao et al., 2002袁洪林等,2003)。

采用Glitter (ver4.0,Macquarie University)程序对锆石的同位素比值及元素含量进行计算,并按照Andersen Tom的方法(Andersen,2002),用LAMICPMS Common Lead Correction (ver3.15)对其进行了普通铅校正,年龄计算及谐和图采用Isoplot (ver3.0)完成(Ludwig,2003)。

2.3 分析结果

对拉轨岗日变质核杂岩核部花岗片麻岩进行了LA-ICPMS锆石U-Pb定年和锆石内部结构阴极发光发光(CL)观察(图 2)。双目镜下显示,所测锆石均大部分为无色透明,少数为浅米黄色,自形程度相对较好,长柱状锆石长宽比多在3:1~2:1。依据锆石阴极发光内部结构的差异,可将锆石大体可以分为两类:第一类锆石从中部至边部发育相对较好的韵律环带(1、4、7、9~11、13~18号锆石),或者锆石中心为CL强度均一,不显环带结构,如12号锆石。第二类锆石数量较少,发育宽缓或窄的振荡环带结构,外围均发育明显的暗色生长加大边,如2、3、5、6、16号锆石。

图 2 拉轨岗日变质核杂岩花岗片麻岩典型锆石阴极发光图像 Fig. 2 Representative zircon CL images of granitic gneiss of Laguigangri metamorphic core complexes

轨岗日变质核杂岩核部花岗片麻岩(07LG-1)的锆石U-Pb分析结果经校正后的有效数据点共18个,从表 1可知,第一类锆石13个测点数据在谐和图中围绕谐和线成群集中分布(图 3),锆石206Pb/238U表面年龄为522±6Ma~503±5Ma,加权平均年龄为514±3Ma,二者在误差范围内基本一致,Th/U比值一般为0.25~1.29,结合阴极发光图像特征,属于岩浆结晶锆石。第二类锆石在谐和图中发散分布,3号和5号207Pb/206U表面年龄分别为1333±11Ma和1544±11Ma,其中3号锆石Th/U比值较低,可能受到构造事件、变质或蚀变等因素影响。2、6、16号锆石206Pb/238U表面年龄为770~896Ma,Th/U比值一般为0.40~0.55,可能属于捕获的早期岩浆成因锆石。

表 1 定日县长所乡北拉轨岗日变质核杂岩花岗片麻岩MC-LA-ICP-MS锆石U-Pb同位素测年结果 Table 1 LA-ICP-MS zircon U-Pb isotopic analysis of granitic gneiss of Laguigangri metamorphic core complexes in northern Changsuo village, Dingri County

图 3 拉轨岗日变质核杂岩花岗片麻LA-ICP-MS锆石U-Pb年龄谐和图 Fig. 3 LA-ICP-MS zircon U-Pb concordia diagrams of granitic gneiss of Laguigangri metamorphic core complexes

锆石内部结构差异较大,可能影响因素很多,如岩浆结晶过程中岩浆动力学变化、晶体-岩浆界面特征的差异、熔体中元素的饱和度、元素扩散等(Corfu et al., 2003Black et al., 2003)。本次所测锆石内部结构不均一,具有明显深熔成因锆石的特征,可能是锆石结晶过程中深熔流体作用的结果(Song et al., 2010)。

综上所述,1544±11Ma~788±8Ma的锆石表面年龄可以解释为岩浆部分熔融、运移及结晶作用过程中捕获的围岩年龄信息。514±3Ma应该为轨岗日变质核杂岩核部花岗片麻岩的形成年龄。

3 花岗片麻岩岩石地球化学特征 3.1 主量元素地球化学特征

拉轨岗日变质核杂岩花岗质片麻岩主量元素分析见表 2。高SiO2 (71.57%~74.20%)、K2O (3.02%~5.90%);Al2O3为11.11%~12.64%;Na2O为2.09%~2.54%,K2O/Na2O为1.27~2.32,属于钾质岩石;低Fe2O3(0.32%~0.50%)、MgO (0.78%~1.27%)、MnO (0.04%~0.10%);FeOT/MgO为2.20~3.89,平均3.13,比值稍高于一般I型、M型花岗岩(Whalen et al., 1987),低于A型花岗岩(Turner et al., 1992)。P2O5变化于0.11~0.14,与高分异S型花岗岩接近(P2O5=0.14),明显不同于A型花岗岩(King et al., 1997)。铝饱和指数A/CNK为1.07~1.19,平均1.12,在A/NK-A/CNK图解中样品均落入过铝质区(图 4),且CIPW标准矿物计算所有样品均含有刚玉(C),含量为1.01%~2.37%,显示强过铝质岩石的特征。里特曼指数(K2O+ Na2O)2/(SiO2-43)为1.02~1.84,为钙碱性系列,在SiO2-K2O图解上样品投入高钾钙碱性系列区(图 5)。在Harker图解中所有样品中的Al2O3、CaO、MgO、FeOT、TiO2随着SiO2含量的增加而减少,K2O随着SiO2的含量增加而增加,均呈较好的线性关系(图 6),反映其原始岩浆具有同源或同时代的特征。

表 2 拉轨岗日变质核杂岩花岗片麻岩主量元素含量分析结果(wt%) Table 2 Major element compositions of granitic gneiss of Laguigangri metamorphic core complexes (wt%)

图 4 拉轨岗日变质核杂岩花岗片麻岩A/NK-A/CNK图解(据Maniar and Piccoli, 1989) Fig. 4 A/NK-A/CNK diagram of granitic gneiss of Laguigangri metamorphic core complexes (after Maniar and Piccoli, 1989

图 5 拉轨岗日变质核杂岩花岗片麻岩SiO2-K2O图解(据Rickwood, 1989) Fig. 5 SiO2-K2O diagram of granitic gneiss of Laguigangri metamorphic core complexes (after Rickwood, 1989)

图 6 拉轨岗日变质核杂岩花岗片麻岩的Harker图解 Fig. 6 Harker diagram of granitic gneiss of Laguigangri metamorphic core complexes

综上表明,拉轨岗日花岗质片麻岩总体上高SiO2、K2O,低Fe2O3、MgO、MnO,A/CNK大部分大于1.1,含过铝质矿物白云母、石榴子石及刚玉,为高钾钙碱性过铝质岩石,与典型的S型花岗岩特征相似(邱家骧和林景仟,1991Chappell and White, 2001李献华等,2007)。

3.2 稀土和微量元素地球化学特征

表 3可知,拉轨岗日花岗片麻岩稀土总量(∑REE=273.7×10-6~481.3×10-6)较高,(La/Yb)N=5.89~16.66,在球粒陨石标准化的稀土元素配分曲线表现为轻稀土富集型(图 7a)。(La/Sm)N=3.12~3.85,(Gd/Yb)N=1.29~2.63,表明存在比较明显的轻稀土分馏。Eu负异常(Eu=0.31~0.39),表明拉轨岗日花岗片麻岩成岩时熔浆体系中缺少斜长石,也是S型花岗岩的特点之一。在原始地幔标准化的微量元素蛛网图上(表 4图 7b),岩石富集大离子亲石元素Rb、Th、U,贫高场强元素Sr、Nb,Hf (两个样品)及Ba等元素为特征,同样具有S型花岗岩的特点(Pearce et al., 1984肖庆辉等,2002林慈銮等,2006雍拥等,2008)。

表 3 拉轨岗日变质核杂岩花岗片麻岩稀土元素含量(×10-6)分析结果 Table 3 REE element compositions (×10-6) of granitic gneiss of Laguigangri metamorphic core complexes

表 4 拉轨岗日变质核杂岩花岗片麻岩微量元素含量(×10-6)分析结果 Table 4 Trace element compositions (×10-6) of granitic gneiss of Laguigangri metamorphic core complexes

图 7 稀土元素球粒陨标准化配分图解(a)和微量元素原始地幔标准化蛛网(b)(标准化值据Sun and McDonough, 1989) Fig. 7 Chondrite-normalized REE patterns (a) and primitive-mantle normalised spidergram (b) for the granitic gneiss of Laguigangri metamorphic core complexes (normalized data from Sun and McDonough, 1989)
3.3 同位素地球化学特征

拉轨岗日花岗质片麻岩的Sr、Nd、Pb同位素组成见表 5,根据岩浆结晶年龄t=514Ma对岩石87Sr/86Sr初始比值ISrεNd(t)、Pb同位素等比值进行统一计算。拉轨岗日片麻状花岗岩87Sr/86Sr初始比值ISr为0.709~0.7306,平均值(0.716)与现代大陆硅铝质岩的ISr平均值为0.719接近,明显高于未受地壳硅铝物质物质混染的大洋玄武岩ISr值(0.702~0.707),ISr指示成岩物质源于地壳(肖庆辉等,2002)。较低的εNd(t)=-7.13~-8.97,与后碰撞强过铝质花岗岩εNd(t)值相似(White and Chappell, 1988),且εNd(t)和ISr无明显相关关系(图 8),表明基本无地幔物质的加入(朱炳泉,1998)。高εSr(t)值(80.65~379),周泰禧等(1996)Chen et al. (2000)研究认为,高εNd(t)值反映了地幔的贡献,而高εSr(t)值反映了地壳物质的贡献。(207Pb/204Pb)t=15.6713~15.7901,比值较高,接近大陆平均值(207Pb/204Pb)t=15.76±0.09,Asmerom et al., 1991)。(206Pb/204Pb)t=18.1062~18.8085。在206Pb/204Pb-207Pb/204Pb图解上(图 9),样品位于上地壳演化线附近,属于上地壳特征的Pb同位素组成。

表 5 拉轨岗日变质核杂岩花岗片麻岩Sr、Nd、Pb同位素组成 Table 5 Sr-Nd-Pb isotope data for granitic gneiss of Laguigangri metamorphic core complexes

图 8 拉轨岗日变质核杂岩花岗片麻岩εNd(t)-ISr图解 Fig. 8 εNd(t)-ISr diagram for Laguigangri metamorphic core complexes

图 9 Pb同位素演化图解(底图据Zartman et al., 1981) Fig. 9 Diagram of Pb isotope for Laguigangri metamorphic core complexes (after Zartman et al., 1981)
4 岩浆起源及源区性质探讨

根据Sylvester (1998)研究表明,CaO/Na2O比值能反映SiO2含量介于67%~77%的过铝质花岗岩源区成分特征,由砂岩部分熔融形成的花岗质熔体的CaO/Na2O比值高于由泥岩部分熔融形成的熔体,泥岩生成的过铝质花岗岩所含的CaO/Na2O比一般小于0.3,而砂屑岩生成的过铝质花岗岩所含的CaO/Na2O比一般大于0.3。拉轨岗日片麻状花岗岩的CaO/Na2O比值介于0.46~0.71,高于0.3,说明其源岩可能为砂岩,源区成熟度相对不高。过铝质花岗岩Rb-Sr-Ba含量变化与它们源岩中起作用的泥质岩及砂屑岩的源区一致。因此,Rb-Sr-Ba系统比值的变化可以很好判断源区成分(Sylvester,1998)。在Rb/Sr-Rb/Ba图解中(图 10a)显示Rb/Sr随Rb/Ba增长而增长的线性关系,而且大多数分布在富粘土源岩区,仅一个样品位于贫粘土源岩区;在Al2O3/(MgO+FeOT)-CaO/(FeOT+MgO)图解上(图 10b)大部分样品仍投在泥质岩源岩区,仅一件样品落入杂砂岩区。以上说明拉轨岗日花岗质片麻岩物源具有不均一性,介于泥岩质和砂岩质之间,可能以粘土岩主,砂质岩占次要地位。

图 10 拉轨岗日变质核杂岩花岗片麻岩的Rb/Ba-Rb/Sr图解(a,据Sylvester,1998)和Al2O3/(FeOT+MgO)-CaO/(FeOT+MgO)图解(b,据Alther et al., 2000) Fig. 10 Diagrams of Rb/Ba vs. Rb/Sr (a, after Sylvester, 1998) and Al2O3/(MgO+FeOT) vs. CaO/(FeOT+MgO) (b, Gerdes et al., 2000) for Laguigangri metamorphic core complexes

拉轨岗日花岗片麻岩具有明显的过铝质特征(A/NKC>1.07),与后碰撞强过铝花岗岩石学及地球化学特征相似,含有高铝矿物及刚玉,相对高的SiO2含量,地壳特征的Sr、Nd、Pb同位素组成及本区缺少同源的中基性岩浆活动等特征,就排除了其直接来自地幔源区的可能性。地球演化过程中K、Rb不断向上迁移进人硅铝层,上地幔越来越亏损K、Rb,Sr主要富集在斜长石中代替Ca的位置。因此,Rb/Sr比值越高,说明源岩主要来自上部陆壳,且上部陆壳的Rb/Sr值大约为0.32,大陆壳平均Rb/Sr值大约为0.24(Taylor and Mcleannan, 1985)。研究区片麻状花岗岩的Rb/Sr值远大于0.32,据此可以判断拉轨岗日过铝质片麻状花岗源岩为上部陆壳,与同位素地球化学特征一致。

温度升高使含钛矿物(如黑云母、钛铁矿)更易分解,更多的TiO2进入融浆,因而Al2O3/ TiO2比值为源岩形成的温度提供了判别标志,Al2O3/TiO2>100为低温( < 875℃),Al2O3/TiO2 < 100为高温( > 875℃)(Sylvester,1998)。本区花岗质片麻岩Al2O3/ TiO2比值均小于24,反映其部分熔融温度为高温,与拉轨岗日花岗片麻岩源岩含有杂砂岩熔融所需的高温条件相一致。

Sr、Nd同位素组成表明,拉轨岗日花岗质片麻岩具有较负εNd(t)值和很老的Nd模式年龄(tDM=2.02~2.30Ga,表 5),表明其来源可能是古老的上地壳物质,可能与古元古代古老地壳物质的再循环有关。结合87Sr/86Sr初始比值特征,综上推断拉轨岗日花岗质片麻岩源岩形成的深度较浅,可能以粘土岩为主,砂质岩占一定比例的沉积岩在高温环境下部分熔融形成的花岗质岩浆。

5 构造环境分析

过铝质花岗岩曾研究认为是陆-陆碰撞过程中同碰撞早期挤压环境下地壳加厚而发生部分熔融的产物(Pitcher,1983Pearce et al., 1984Harris et al., 1986),但在碰撞高峰期后的岩石圈伸展背景下仍能产生过铝质花岗岩(Sylvester,1998Williamson et al., 1996Kalsbeek,2001)。拉轨岗日花岗质片麻岩属高钾钙碱性系列,而高钾钙碱性系列岩浆岩是后碰撞岩浆活动的重要特征之一(Zhao et al., 1996Liégeois et al., 1998),在花岗岩类R1-R2因子判别图上(图 11a),拉轨岗日花岗质片麻岩样品基本投在了同碰撞花岗岩区。利用Rb-(Y+Nb)图解(图 11b),花岗片麻岩样品的投点均落在后碰撞花岗岩区,并由从同碰撞花岗岩区向非造山的板内花岗岩区域过渡的趋势,在FeOT/(FeOT+MgO)-SiO2图解上(图 11c),除一个样品外,其它投在后造山花岗岩类区,说明拉轨岗日花岗质片麻岩形成于构造体制转换的地球动力学背景下,由主碰撞的挤压环境向后碰撞的伸展环境转化阶段,在这一过程中降压和升温的环境可能是岩石发生熔融的主要因素(吴福元等,2007)。结合区域资料,拉轨岗日杂岩及其上部的抗青大岩组原岩为一套石英砂岩、灰岩和富泥细砂-泥质粉砂岩组合,含玄武岩或玄岩质岩脉,与高喜马拉雅基底变质岩的原岩建造基本上可进行对比(李德威等,2003),均代表了一种稳定伸展环境的产物。刘文灿等(2002)研究认为奥陶纪拉轨岗日构造带并不存在明显的挤压造山作用,而表现为伸展-拉张构造环境。综上说明拉轨岗日花岗质片麻岩形成于后碰撞造山的构造环境,主碰撞造山阶段结束。该阶段包括了板块之间沿剪切带的大规模运动,岩石圈拆沉与热软流圈的上涌等,致使地壳受热诱发部分熔融(肖庆辉等,2002),拉轨岗日花岗质片麻岩源岩可能是在这种条件下形成的。

图 11 拉轨岗日变质核杂岩花岗片麻岩构造环境判别图解(a,据Batchelor and Bowden, 1985;b,据Pearce et al., 1984;c,据Maniar and Piccoli, 1989) VAG-火山弧花岗岩;Syn-COLG-同碰撞花岗岩;WPG-板内花岗岩;ORG-洋脊花岗岩;Post-COLG-后碰撞花岗岩;RRG-与裂谷有关的花岗岩类;CEUG-与造陆抬升有关的花岗岩类;POG-后造山花岗岩类;IAG-岛弧花岗岩类;CAG-大陆弧花岗岩类;CCG-大陆碰撞花岗岩类 Fig. 11 Diagrams of the tectonic setting for Laguigangri metamorphic core complexes (a, after Batchelor and Bowden, 1985; b, after Pearce et al., 1984; c, after Maniar and Piccoli, 1989)
6 大地构造意义

东冈瓦纳的形成及其与西冈瓦纳大陆的结合是一系列复杂造山事件的结果,被称为泛非造山作用(Knenedy,1964Kroner,1984),时间跨度从~750Ma到~510Ma (Meert,2003Cawood et al., 2007)。自从Gansser (1964)提出喜马拉雅造山带形成于早古生代的观点以来,泛非造山作用与喜马拉雅造山带的关系成为研究的焦点。

中外地质学家在喜马拉雅及以北和以东地区相继发现了大量泛非期构造-热事件的地质记录(Foster,2000Gehrels et al., 2003刘文灿等,2004aSong et al., 2007夏斌等,2008李才等,2008)。许志琴等(2005)对喜马拉雅基底变质岩系进行了系统研究,获得529~457Ma年代学数据,认为原始的喜马拉雅山是泛非-早古生代造山事件的产物;Liu et al.(2006)在高喜马拉雅亚东地区结晶岩中获得~500Ma的岩浆事件年龄,认为亚东地区属泛非造山带的延伸部分之一。此外,朱同兴等(2003)在北喜马拉雅札达-吉隆微地块的定日县南发现了一套盖层底砾岩残留体,早古生代沉积物以超覆接触形式沉积在变质岩基底之上。周志广等(2004)在北喜马拉雅康马-隆子微地块的康马县康马岩体西南侧奥陶系底部发现一套底砾岩,不整合于中-新元古代拉轨岗日岩群之上;程立人等(2005)在冈底斯-念青唐古拉地块的申扎县塔尔玛乡木纠错地区确立了下奥陶统他多组与前震旦系念青唐古拉岩群的不整合关系等均为泛非期造山事件的不整合证据。

以上说明泛非构造-岩浆事件在喜马拉雅一带普遍存在。本次通过高精度的锆石U-Pb测年,获得514Ma的岩浆结晶年龄,与前人研究成果吻合,是泛非构造-热事件在北喜马拉雅拉轨岗日一带的地质记录,说明在古生代早期北喜马拉雅拉轨岗日一带卷入了泛非事件中。

研究显示冈瓦纳大陆北缘喜马拉雅和冈底斯带是全球泛非运动持续时间最长、结束最晚(490~485Ma)的大陆边缘地区(周志广等,2004Song et al., 2007),且藏南、藏北具有统一约500Ma的泛非基底,该变质基底是全球范围内冈瓦纳大陆统一变质基底形成最晚的区域(任纪舜和肖黎薇,2004)。通过岩石地球化学研究认为拉轨岗日花岗质片麻岩形成于挤压环境向伸展环境转变的后碰撞造山环境,结合新得到的514Ma同位素年龄,说明该时期泛非碰撞造山事件在拉轨岗日一带可能结束,进入后碰撞造山的构造演化阶段。后碰撞造山阶段岩石圈拆沉与软流圈的上涌等热动力作用引起地壳伸展减薄和熔融,岩浆侵入可能使拉轨岗日变质核杂岩核部岩石初步隆起。后期岩浆活动促使内核的再次上隆起,对拉轨岗日变质核杂岩的最终形成起了重要作用。

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